Heat the oven to 400°F. Stir the beans and picante sauce in a small bowl.
Place 6 tortillas onto 2 baking sheets. Spread about 1/3 cup bean mixture on each tortilla to within 1/2-inch of the edge. Top with the cheese and onions. Brush the edges of the tortillas with water. Top with the remaining tortillas and press to seal.
Bake for 10 minutes or until the filling is hot. Cut each quesadilla into 4 wedges. Serve with additional picante sauce.